Reducing solution for silvering operations



Patented Apr. 22, MM

REDUCING SOLUTION FDR SHINE UPERA'EIONS to Pittsburgh Plate Glass Com Coty, ra, a corporation oi E No wing. Application Serial No. 198,608

a Claims. (01. assiss) The present invention relates to reducing solutions employed in the deposition of metals from their salts and more particularly to solutions.

which are adapted for use in the silverlng of mirrors.

One object of the present invention is the provision of a reducing solution which has a relatively slow rate of reaction with silver oxide, and which, therefore, will result in the deposition of an exceptionally uniform and continuous silver film in the plating of mirrors.

Other objects and advantages of-my invention will become more apparent from. the following detailed description or certain embodiments thereof.

The deposition of silver films upon glass plates- .of this composition, however, reacts very rapidly with the silvering solution and the reduced silver oxide is deposited upon the glass plates as an imperfect fllm. Such rapid deposition prevents a good adhesion of the silver to the gl and the resultant mirror is subject to-early deteriora- Various attempts have been made to control the rate of reaction but they have not'prov'en satisfactory. For example, the concentration of the ammoniacal silver solution has been reduced but the dilute solutions require a longer time for the deposition or an extremely thin film of silver. In other cases, the silvering has been performed at lower temperatures; an expedient which would reduce the rate of deposition to he sure, but which at the same time ted in the formation of porous, imperfect silver It has been found that the addition or a relatlvely small percene of a water soluble, organic acid to the reducing sugar solution de creases the speed of the deposition of silver at. higher operating temperatur' Formlc, acetic, propionic, citric, and tartaric acids may be used 7 as the additive produg the desired efiects. an

ceptionally fine r have been obtained 'th h a or the indicated acids and others fall-- ing v vithi r i the scope of the designated class may be substituted therefor.

It is believed that the organic acid acts m the nature of aninhibitor or negative catalyst of the reducing action by changing the hydrogen ion concentration of the sugar solution, retarding the decomposition of the ammoniacal solution of silver oxide and the subsequent deposition of metal- 10 he silver. This control over the reducing solution permits a regulation of the rate of silvering while the other conditions under which the plating operation is performed are maintained for optimum .efllciency,

The reducing sugar solution is readily. prepared by the addition of the water soluble, organic I acid, dextrose and aliphatic acid to distilled water. The formulation may be illustrated as:

Per cent by weight Dextrose a. 1 to 5 Denatured ethyl alcohol 8 to 8 Water soluble organic acid 0.5 to 10 Distilled water.-. 7'? to 96 Other reducing sugars, including invert sugar,

could be substituted for dextrose, andmethyl or propyl alcohol could replace the ethyl alcohol orbe'used in combination therewith.

As a more specific example of a reducing solu- 6 tion as contemplated by my invention, the following composition is given:

Per cent by weight Dextrose 2 Denatured ethyl alcohol a 5 Tartar-lo acld. 1 Distilled water 93 Thesolution is prepared according to standard .practice, that is, the reducing sugar is dissolved in a small amount of water and subsequently the 40 other components and the remainder of the wa- 5 thoroughly and placed on a level table, which may be heated to about 80 degrees F. The silvering solution is prepared by mixing anammoniacal solution of silver oxide with an equivalent amount of the reducing solution and is then poured over the glass plate. After. the silver film has been deposited on the glass, any excess of reagents is removed, the .glass is-washed and dried and the silver film is then coated with a protective coatn8- through the use 05 tartaric and citric acids, al-= The acid inhibited reducing solution as contemplated by my inventionenables the producof ethyl alcohol, from 1 tion of superiorsilver films due to the retarded rate of deposition and the greater controlpossible over the reaction. The silver films are strongly adhered to the glass and will, therefore, resist deterioration.

It will be obvious that various modifications may be made in the particular compositions disclosedand the manner without departing from tion or the scope of the WhatIclaim is:

'1. A reducing sugar solution for use in 'the silvering of mirrors comprising a water soluble organic acid selected from the group consisting of acetic, citric and tartaric acids, an aliphatic alcohol and a reducing sugar.

2. A reducing sugar solution for use in the silvering of mirrors comprising from 0.5 to 10 per cent by weight of a water soluble, organic acid selected from the group consisting of acetic, citric and tartaric acids, approximately 'per cent by weight of an aliphatic alcohol, from 1 to 5 per cent by weight 01' a reducing sugar, and distilled water.

in which they are used the spirit of my invenappended claims.

3. A reducing sugar solution for use in the s lvering' of mirrors comprising from 0.5 to per cent of tartaric acid, approximately 5 per cent of ethyl' alcoho from 1 1305 per cent by weight of dextrose, and distilled water.

4. A reducing sugar solution for use in the silvering' of mirrors comprising from 0.5 to 10 per cent of citric acid, approximately 5 per cent to 5 per cent by weight of invert sugar, and distilled water.

5. A reducing sugar solution for use in the silvering of mirrors having the approximate composition:

Per cent Dextrose 2 Denatured ethyl alcohol 4 Tartaric acid- 4 1 5 Distilled water; 93 6. A reducing sugar solution for use in the silvering of mirrors having the approximate composition:

, Per cent 10 Invert sugar Denatured; ethyl alcohol 4 Tartaric arid 1 Distilled water 93 7. A reducing sugar solution for use in the silvering of mirrors having the approximate composition:

Per cent Dextrose 2 Denatured ethyl alcohol 4 Acetic acid l Distilled water 93 8. A process of preparing metallic silver from a solution of ammoniacal silver oxide which comprises adding thereto an aqueous solution of a reducing sugar containing from 0.5 to 10 per cent of a water soluble organic acid.

9. A process of preparing metallic silver from a solution of ammoniacal silver oxide comprising adding thereto an aqueous solution including in approximate amounts 5 per cent of dextrose, 5 per cent of ethyl alcohol and 1 per cent of tartaric acid, the tartaric acid content of the reducing sugar solution serving to retard the rate at which the ammoniacal silver oxide is converted into metallic silver. 

